RXRα Positively Regulates Expression of the Chicken PLIN1 Gene in a PPARγ-Independent Manner and Promotes Adipogenesis

Perilipin1 (PLIN1), the most abundant lipid droplet (LD)-associated protein, plays a vital role in regulating lipid storage and breakdown in adipocytes. Recently, we found that the overexpression of PLIN1 promotes chicken preadipocyte lipid accumulation. However, the mechanisms by which transcription of the chicken PLIN1 gene is regulated remain unknown. In this study, we investigated the role of retinoid X receptor α (RXRα) in transcription of the chicken PLIN1 gene. Notably, reporter gene and expression assays showed that RXRα activates transcription of the chicken PLIN1 gene in a PPARγ-independent manner. Furthermore, promoter deletion and electrophoretic mobility shift assay (EMSA) analysis revealed that the chicken PLIN1 gene promoter region (-774/-785) contains an RXRα-binding site. Further study demonstrated that RXRα overexpression promotes differentiation of an immortalized chicken preadipocyte cell line (ICP1), causing a concomitant increase in PLIN1 transcripts. Taken together, our results show for the first time that RXRα activates transcription of the chicken PLIN1 gene in a PPARγ-independent manner, which might be at least in part responsible for RXRα-induced adipogenesis.

Expression of chicken ovalbumin upstream promoter-transcription factor II and liver X receptor as prognostic indicators for human colorectal cancer

Cholesterol increases the risk of colorectal cancer. Liver X receptor (LXR), retinoid X receptor (RXR)α and sterol regulatory element binding protein (SREBP)-1c are transcriptional regulators of lipid metabolism. Chicken ovalbumin upstream promoter-transcription factor II (COUP-TFII) serves an essential role in angiogenesis and development, but its role in cancer is controversial. The expression of COUP-TFII, LXR, RXRα and SREBP-1c in colorectal cancer, as well as their association with clinicopathologic features, was assessed, and their utility as prognostic indicators in colorectal cancer evaluated. Colorectal cancer samples (n=707 patients) were analyzed for COUP-TII, LXR, RXRα and SREBP-1c expression by immunohistochemistry. Overall survival curves of patients with tumors expressing different levels of these proteins were produced and risk factors were assessed. Of the 707 patients, 32.7, 50.9, 56.4, and 41.7% were positive for COUP-TFII, LXR, RXRα, and SREBP-1c, respectively. The lack of COUP-TFII or LXR expression was associated with lower overall survival rates (P=0.0154 for COUP-TFII, and 0.0113 for LXR). Following adjustment for other clinical risk factors (age, sex, tumor size, grade, vascular invasion, and Tumor-Node-Metastasis stage), the lack of COUP-TFII or LXR expression was a negative independent prognostic factor for survival. The expression of COUP-TFII and LXR alone or in combination may be biomarkers to indicate a positive prognosis in patients with colorectal cancer.

Retinoid X receptor α enhances human cholangiocarcinoma growth through simultaneous activation of Wnt/β-catenin and nuclear factor-κB pathways

Retinoid X receptor α (RXRα) plays important roles in the malignancy of several cancers such as human prostate tumor, breast cancer, and thyroid tumor. However, its exact functions and molecular mechanisms in cholangiocarcinoma (CCA), a chemoresistant carcinoma with poor prognosis, remain unclear. In this study we found that RXRα was frequently overexpressed in human CCA tissues and CCA cell lines. Downregulation of RXRα led to decreased expression of mitosis‐promoting factors including cyclin D1and cyclin E, and the proliferating cell nuclear antigen, as well as increased expression of cell cycle inhibitor p21, resulting in inhibition of CCA cell proliferation. Furthermore, RXRα knockdown attenuated the expression of cyclin D1 through suppression of Wnt/β‐catenin signaling. Retinoid X receptor α upregulated proliferating cell nuclear antigen expression through nuclear factor‐κB (NF‐κB) pathways, paralleled with downregulation of p21. Thus, the Wnt/β‐catenin and NF‐κB pathways account for the inhibition of CCA cell growth induced by RXRα downregulation. Retinoid X receptor α plays an important role in proliferation of CCA through simultaneous activation of Wnt/β‐catenin and NF‐κB pathways, indicating that RXRα might serve as a potential molecular target for CCA treatment.

Retinoid X receptor α overexpression alleviates mitochondrial dysfunction-induced insulin resistance through transcriptional regulation of insulin receptor substrate 1

Mitochondrial dysfunction is associated with insulin resistance and diabetes. We previously showed that retinoid X receptor α (RXRα) played an important role in transcriptional regulation of oxidative phosphorylation (OXPHOS) genes in cells with mitochondrial dysfunction caused by mitochondrial DNA mutation. In this study, we investigated whether mitochondrial dysfunction induced by incubation with OXPHOS inhibitors affects insulin receptor substrate 1 (IRS1) mRNA and protein levels and whether RXRα activation or overexpression can restore IRS1 expression. Both IRS1 and RXRα protein levels were significantly reduced when C2C12 myotubes were treated with the OXPHOS complex inhibitors, rotenone and antimycin A. The addition of RXRα agonists, 9-cis retinoic acid (9cRA) and LG1506, increased IRS1 transcription and protein levels and restored mitochondrial function, which ultimately improved insulin signaling. RXRα overexpression also increased IRS1 transcription and mitochondrial function. Because RXRα overexpression, knock-down, or activation by LG1506 regulated IRS1 transcription mostly independently of mitochondrial function, it is likely that RXRα directly regulates IRS1 transcription. Consistent with the hypothesis, we showed that RXRα bound to the IRS1 promoter as a heterodimer with peroxisome proliferator-activated receptor δ (PPARδ). These results suggest that RXRα overexpression or activation alleviates insulin resistance by increasing IRS1 expression.

SREBP-2 negatively regulates FXR-dependent transcription of FGF19 in human intestinal cells

Sterol regulatory element-binding protein-2 (SREBP-2) is a basic helix-loop-helix-leucine zipper transcription factor that positively regulates transcription of target genes involved in cholesterol metabolism. In the present study, we have investigated a possible involvement of SREBP-2 in human intestinal expression of fibroblast growth factor (FGF)19, which is an endocrine hormone involved in the regulation of lipid and glucose metabolism. Overexpression of constitutively active SREBP-2 decreased FGF19 mRNA levels in human colon-derived LS174T cells. In reporter assays, active SREBP-2 overexpression suppressed GW4064/FXR-mediated increase in reporter activities in regions containing the IR-1 motif (+848 to +5200) in the FGF19 gene. The suppressive effect disappeared in reporter activities in the region containing the IR-1 motif when the mutation was introduced into the IR-1 motif. In electrophoretic mobility shift assays, binding of the FXR/retinoid X receptor α heterodimer to the IR-1 motif was attenuated by adding active SREBP-2, but SREBP-2 binding to the IR-1 motif was not observed. In chromatin immunoprecipitation assays, specific binding of FXR to the IR-1-containing region of the FGF19 gene (+3214 to +3404) was increased in LS174T cells by treatment with cholesterol and 25-hydroxycholesterol. Specific binding of SREBP-2 to FXR was observed in glutathione-S-transferase (GST) pull-down assays. These results suggest that SREBP-2 negatively regulates the FXR-mediated transcriptional activation of the FGF19 gene in human intestinal cells.

Activation of farnesoid X receptor induces RECK expression in mouse liver

Farnesoid X receptor (FXR) belongs to the ligand-activated nuclear receptor superfamily, and functions as a transcription factor regulating the transcription of numerous genes involved in bile acid homeostasis, lipoprotein and glucose metabolism. In the present study, we identified RECK, a membrane-anchored inhibitor of matrix metalloproteinases, as a novel target gene of FXR in mouse liver. We found that FXR agonist substantially augmented hepatic RECK mRNA and protein expression in vivo and in vitro. FXR regulated the transcription of RECK through directly binding to FXR response element located within intron 1 of the mouse RECK gene. Moreover, FXR agonist reversed the down-regulation of RECK in the livers from mice fed a methionine and choline deficient diet. In summary, our data suggest that RECK is a novel transcriptional target of FXR in mouse liver, and provide clues to better understanding the function of FXR in liver.

Alteration of cardiac glucose metabolism in association to low birth weight: experimental evidence in lambs with left ventricular hypertrophy

OBJECTIVE

Intrauterine growth restriction that results in low birth weight (LBW) has been linked to the onset of pathological cardiac hypertrophy. An altered transition from a fetal to an adult energy metabolism phenotype, with increased reliance on glucose rather than fatty acids for energy production, could help explain this connection. We have therefore investigated cardiac metabolism in relation to left ventricular hypertrophy in LBW lambs, at 21days after birth.

MATERIALS/METHODS

The expression of regulatory molecules involved in cardiac glucose and fatty acid metabolism was measured using real-time PCR and Western blotting. A section of the left ventricle was fixed for Periodic Acid Schiff staining to determine tissue glycogen content.

RESULTS

There was increased abundance of insulin signalling pathway proteins (phospho-insulin receptor, insulin receptor and phospho-Akt) and the glucose transporter (GLUT)-1, but no change in GLUT-4 or glycogen content in the heart of LBW compared to ABW lambs. There was, however, increased abundance of cardiac pyruvate dehydrogenase kinase 4 (PDK-4) in LBW compared to ABW lambs. There were no significant changes in the mRNA expression of components of the peroxisome proliferator activated receptor regulatory complex or proteins involved in fatty acid metabolism.

CONCLUSION

We concluded that LBW induced left ventricular hypertrophy was associated with increased GLUT-1 and PDK-4, suggesting increased glucose uptake, but decreased efficacy for the conversion of glucose to ATP. A reduced capacity for energy conversion could have significant implications for vulnerability to cardiovascular disease in adults who are born LBW.

Blockage of progesterone receptor effectively protects pancreatic islet beta cell viability

The progesterone receptor (PR), a member of nuclear receptor superfamily, is closely associated with gestational, type 1 and type 2 diabetes. However, the underlying mechanisms remain obscure. Here we found that PR activation increased the pro-inflammatory cytokines (PIC)-induced injury in Min6 cells, and PR blockage with siRNA interference protected the cells from damage. Moreover, the new discovered PR antagonist SC51089 effectively improved cell survival by reducing the PIC-stimulated cell apoptosis in Min6 cells. Immunoblotting assays indicated that either PR agonist progesterone (P4) or PR-B over-expression promoted the PIC-induced reinforces of extracellular-signal-regulated kinase 1/2 phosphorylation (p-Erk) and protein 53 (p53), and the attenuations of protein kinase B phosphorylation (p-AKT) and tumor necrosis factor receptor-associated factor 2 (TRAF2). SC51089 could reverse all the P4- or PR-B over-expression induced effects. In addition, PR siRNA inference based assay further supported that SC51089 protected pancreatic islet beta cells from the PR activation or PIC-induced injury by targeting PR and this protective action was mediated by AKT signaling pathway. To our knowledge, this current work might be the first report on the regulation of PR in pancreatic islet beta cell survival. It is expected that SC51089, as a non-steroid PR antagonist, might also find its potential in anti-diabetic research.

Identification of 15d-PGJ2 as an antagonist of farnesoid X receptor: molecular modeling with biological evaluation

15-Deoxy-Δ(12,14)-PGJ2 (15d-PGJ2) is one of the major metabolites from prostaglandin D2 in arachidonic acid (AA) metabolic pathway. It was determined as a ligand of peroxisome proliferator-activated receptor γ (PPARγ) functioning potently in adipocyte development. However, the fact that 15d-PGJ2 exerts also PPARγ-independent biological actions has highly addressed its multi-target behavior. Here, we identified that 15d-PGJ2 was an antagonist of farnesoid X receptor (FXR), as investigated by surface plasmon resonance, fluorescence quenching and homo time-resolved fluorescence based analyses, and the coactivator-recruitment and luciferase-reporter related investigation. Assay of 15d-PGJ2 regulation on hFXRα target genes revealed that treatment of HepG2 cells with 15d-PGJ2 resulted in the stimulation of mRNA expressions of bile-salt export pump (BSEP), and the decrease of cholesterol 7a-hydroxylase (CYP7a1). In addition, functional assays indicated that 15d-PGJ2 promoted the conversion of cholesterol to bile acids in HepG2 cells. Moreover, molecular docking combined with molecular dynamics simulation was applied to develop the possible model of 15d-PGJ2 binding to hFXRα ligand binding domain (LBD) at atomic level, and the responsible residues for 15d-PGJ2/hFXRα-LBD interaction were thereby determined, which were further confirmed by SPR assays against hFXRα-LBD site-directed mutations. Given that hFXRα functions potently in the regulation of hepatic bile acid metabolism and lipid/glucose homeostasis, our current work is expected to help better understand the multi-target features of this PGD2 metabolite in biological pathways, and 15d-PGJ2 as a new discovered FXR antagonist might find its potential application in further anti-hypercholesterol research.